This invention relates to a process for the conversion of nitrous oxide (N2O) to nitrogen and oxygen in the presence of a supported metal-containing catalyst. The invention also includes a novel catalyst composition and a method for making the catalyst composition.
Nitrous oxide is a greenhouse and ozone-depleting gas, and is a by-product of adipic and nitric acid manufacturing.
U.S. Pat. No. 5,705,136 discloses a process for the decomposition of nitrogen oxides to nitrogen and oxygen comprising contacting the nitrogen oxides with a mixed oxide catalyst wherein the catalyst comprises a first metal oxide selected from the oxides of Mn, Fe, Co, Ni, Cu, Zn and mixtures thereof on a metal oxide support consisting essentially of MgO, CaO, ZnO, TiO2, MoO3xe2x80x94CoOxe2x80x94Al2O3, ZnOxe2x80x94Al2O3, TiO2xe2x80x94MgO, TiO2xe2x80x94Al2O3, TiO2xe2x80x94ZnO, MgOxe2x80x94CuO and MgOxe2x80x94NiO or mixtures thereof.
U.S. Pat. No. 5,314,673 discloses a process for the conversion of N2O to nitrogen and oxygen which comprises contacting the N2O with a catalyst consisting essentially of nickel oxide and cobalt oxide on a zirconia substrate.
There is a need for catalysts which can decompose N2O into N2 and O2, and have a minimal environmental impact of their own. That is, they should contain readily-available and non-toxic materials, be simple to make, have a long lifetime, and not pose disposal problems. The catalysts should also be hard and porous.
This invention provides a process for the conversion of nitrous oxide (N2O)) into nitrogen (N2) and oxygen (O2) comprising contacting N2O with a metal-containing catalyst supported on zirconia under conditions effective to decompose the N2O to N2 and O2, wherein the catalyst comprises iron and optionally at least one metal selected from the group consisting of cobalt, nickel, rhodium, palladium, iridium, platinum, manganese, lanthanum and cerium and the catalyst is prepared by the steps of:
(a) preparing a paste comprising contacting zirconium hydroxide with a solution of an iron salt and a zirconium salt, optionally in the presence of binders and lubricants;
(b) forming a shaped particle from the step (a) paste;
(c) drying the step (b) shaped particle;
(d) calcining the dried step (c) shaped particle at a temperature of at least 400xc2x0 C.; and
(e) optionally adding at least one metal selected from the group consisting of cobalt, nickel, rhodium, palladium, iridium, platinum, manganese, lanthanum and cerium, to step (a) or to the calcined step (d) shaped particle.
In another embodiment, this invention provides a catalyst composition useful in a process for the decomposition of nitrous oxide, wherein the composition comprises a metal-containing catalyst supported on a zirconia shaped particle, wherein the metal comprises iron and optionally at least one metal selected from the group consisting of cobalt, nickel, rhodium, palladium, iridium, platinum, manganese, lanthanum and cerium, wherein the catalyst is prepared by the steps of:
(a) preparing a paste comprising contacting zirconium hydroxide with a solution of an iron salt and a zirconium salt, optionally in the presence of binders and lubricants;
(b) forming a shaped particle from the step (a) paste;
(c) drying the step (b) shaped particle;
(d) calcining the dried step (c) shaped particle at a temperature of at least 400xc2x0 C.; and
(e) optionally adding at least one metal selected from the group consisting of cobalt, nickel, rhodium, palladium, iridium, platinum, manganese, lanthanum and cerium, to step (a) or to the calcined step (d) shaped particle; and wherein the crush strength of the calcined shaped particle is at least 22.2 newtons.
Zirconium hydroxide (i.e., xe2x80x9cZr(OH)4xe2x80x9d, sometimes referred to as zirconium oxyhydroxide or hydrated zirconia) powder is dried before use at about 50xc2x0 C. to 150xc2x0 C., preferably at about 100xc2x0 C. The zirconium hydroxide can be doped with various elements such as Ca, Mg, Si, and La to help maintain a high surface area upon calcination.
The iron and zirconium salts can be chosen from a wide variety of salts, which readily decompose upon calcination to produce iron and zirconium oxides, such as acetates, carbonates, citrates, nitrates, oxalates and chlorides. Surprisingly, even chlorides may be used, although the other salts are preferred. Sulfates and phosphates can also be included in small amounts, as these anions help maintain a high surface area upon calcination. In addition, other components, such as binders and lubricants, can be added to the paste to aid in the shaping process, e.g., extrusion., and provide green strength. The iron in the iron salts can be in either the +2 or +3 oxidation states, with the +3 oxidation state being preferred. The minimum iron content is 0.5% Fe or a minimum iron nitrate content of the pepping solution is 5%. The preferred iron concentration in the catalyst is 1.5% to 7%, with a most preferred iron concentration of about 3% to 4%.
The process of this invention also includes the use of one or more solvents selected from conventional liquid solvents which are inert in the context of the process of the present invention and easily removed by drying (evaporation) and/or by combustion during calcination. These solvents include water; alcohols, such as methanol, ethanol and propanol; ketones, such as acetone and 2-butanone; aldehydes, such as propanol and butanal; and aromatic solvents such as toluene and benzene. Water is the preferred solvent.
The amount of solvent used in preparing the paste of step (a) is an amount that provides a consistency which allows for a shaped particle to be mechanically formed out of the paste, but not so fluid as to fail to hold its form or shape or become sticky and agglomerate with other particles. Typically, the total amount of solvent in the paste is from about 10% to about 30% by weight of the paste.
The paste of the present process may also contain rheology control agents and pore forming agents. Rheology control agents include starches, sugars, glycols, polyols, powdered organic polymers, graphite, stearic acid and its esters. Pore forming agents include graphite, polypropylene or other organic polymer powders, activated carbon, charcoal, starches and cellulose flour. The rheology control agents and pore forming agents (some materials may perform both functions) are well known to those of ordinary skill in the art and are used as necessary to obtain the desired viscosity of the paste or porosity of the formed particle, as the case may be. Typically, any of these may be present in the amount of from about 0.5% to about 20% by weight, preferably, from about 1% to about 10% by weight of the paste.
A formed or shaped particle is then prepared from the paste. Extrusion is the preferred forming technique. The formed particle may have a variety of cross sections such as cylinders, trilobes, and star shaped. The formed particles are air dried under conditions sufficient to form a particle that is not malleable (or soft) or friable. The dried formed particles are then calcined in air or in inert gases such as nitrogen or argon or mixtures thereof at a temperature of from about 400xc2x0 C. to about 650xc2x0 C. The result is a surprisingly hard and porous iron-zirconia formed particle. The crush strength of the shaped particles is at least about 22.2 newtons (5 pounds).
The rheology control agents and pore forming agents incorporated in the paste are removed from the finished shaped particle by a combination of volatilization and combustion during the final steps of drying and calcination of the shaped particle.
In one embodiment of this invention, catalytic metals for the decomposition of nitrous oxide may be incorporated into the step (a) paste or preferably, impregnated on the calcined step (d) shaped particle. At least one metal is selected from the group consisting of cobalt, nickel, rhodium, palladium, iridium, platinum, manganese, lanthanum and cerium. Suitable sources of catalytically active components include both organic and inorganic compounds. Inorganic compounds are preferred for impregnation of the iron-zirconia shaped particle. These compounds include; Co(NO3)2.6H2O, Ni(NO3)2.6H2O, Rh(NO3)3, Na2PdCl4, IrCl3, H2PtCl6, Pd(NH3)4Cl2, Mn(NO3)2, La(NO3)3.6H2O and Ce(NO3)3.6H2O.
The catalytic metals are present in the amount of from about 0.1 weight percent to about 10 weight percent. A preferred catalyst composition contains nickel and cobalt on the iron-zirconia shaped particle. The ratio of nickel to cobalt in the catalyst is from about 0.5:1 to about 3:1.
Nitrous oxide is contacted with a catalyst of this invention. The nitrous oxide may be diluted with other gaseous components such as nitrogen, oxygen, argon and helium. A typical feed gas from an adipic acid plant which uses nitric acid as the oxidant contains about 10 volume % nitrous oxide; however, higher or lower feed rates are practical both for nitrous oxide produced in adipic acid plants and for other nitrous oxide sources, such as produced during the manufacture of nitric acid. Typical flow rates for nitrous oxide from an adipic acid plant may vary from about 30,000 hrxe2x88x921 to about 40,000 hrxe2x88x921. Again, as is true for the feed gas composition, higher or lower space velocities can be used. The reaction temperature depends on a number of factors such as preheat temperature, nitrous oxide concentration, catalyst composition, etc. The present invention is not dependent on reaction pressure.
Since, in the manufacture of adipic acid by the nitric acid oxidation of a mixture of cyclohexanol/cyclohexanone, nitrous oxide is produced as a by-product, the present invention provides a convenient method of decomposing the by-product nitrous oxide. The method involves contacting the nitrous oxide with a catalyst composition of this invention.